There are many applications for combining lasers and other solid-state light sources. In general, when an application requires more power than can be delivered by a single laser source, a common solution is to combine the light from two or more lasers of the same wavelength. Since the additional lasers are in a physically different location, it becomes necessary to combine and stack the laser output beams together, eliminating as much “dead space” as possible in the combined output beam.
When lasers are combined in this manner, it is often desirable to make the combined source as small as possible (i.e., to have the smallest possible etendue) so that the energy of the combined beam can be effectively and efficiently concentrated and transferred to another optical system. Where lasers emit polarized light, a typical solution for their combination is polarization combining using polarizing optical elements and surfaces. Where neither wavelength difference nor polarization states can be exploited for use in combining the laser light, spatial combination must be used. Spatial combining requires positioning the laser sources and redirection optics in a compact, precision arrangement, so that the combined beams can be packed together as closely as possible, providing source energy while maintaining as low etendue as possible.
One application for which spatial combining of multiple sources is of particular interest is in pump excitation for fiber lasers. In a fiber laser, the active gain medium is an optical fiber doped with suitable rare-earth elements. Pump energy can be provided from a number of types of sources, such as using a set of multiple laser diodes that are fiber-coupled to the gain medium. By using multiple pump sources, higher optical power can be directed to the gain medium. The use of multiple laser sources also allows each of the pump lasers to operate at a lower power level for a given amplifier gain, thereby extending the lifetime of the pump lasers and hence the reliability of the amplifier. This also provides some redundancy in the event that one of the pump lasers fails.
Because a single wavelength is needed for pump energy, the individual sources must be closely matched, making laser diodes a practical choice. However, laser diodes do not provide a beam that is circular in cross section, that is, with highly symmetrical energy distribution about a central axis. Instead, the aspect ratio of the output light is highly asymmetric, with markedly different divergence angles in orthogonal directions, generating an output beam whose length (considered to be along the “slow” axis) can be several times its width (along the “fast” axis). This asymmetric characteristic makes it desirable to stack the component output beams as closely together as possible, to form a composite beam with a more nearly symmetric aspect ratio. As a limiting factor, the input optical fiber for accepting the pump energy has a relative small numerical aperture (N.A.), which limits the angular extent of the incoming composite beam and makes it desirable to eliminate as much dead space between component beams as possible.
Among solutions that have been implemented or proposed for combining laser sources for use as pump lasers is a modular pump module with vertically staggered laser diodes and corresponding mirrors. FIGS. 1A and 1B show top and side views, respectively, of a typical pump module 10 of this type. In this approach, each of three lasers 12a, 12b, and 12c directs a beam through a corresponding cylindrical lens 14a, 14b, and 14c and to a mirror 16a, 16b, and 16c, respectively. A filter 30 provides a measure of protection from feedback light FB, as described in more detail subsequently. A composite beam 28 is then focused by a lens 18 into an optical fiber 20 for use as pump energy. There is also an additional lens on the end of each laser 12a, 12b, and 12c, not shown in these figures.
As the side view of FIG. 1B shows, with vertical distance intentionally exaggerated for clarity, the lasers 12a, 12b, and 12c and their corresponding cylindrical lenses 14a, 14b, and 14c and mirrors 16a, 16b, and 16c are vertically staggered. This arrangement of reflective components leaves little tolerance room between the component output beams. The light from laser 12a is clipped by the top of mirror 16b, for example. Similarly, the light from laser 12b is clipped by the top of mirror 16c. The inset W in FIG. 1A shows how composite beam 28 is formed, with an output beam 22a from laser 12a, an output beam 22b from laser 12b, and an output beam 22c from laser 12c. There is necessarily some dead space 24 between the output beams due to tolerances needed to pass the beams by the fold mirrors 16b and 16c. 
While the solution described with reference to FIGS. 1A and 1B has proved to be workable, there is room for improvement. Manufacturing tolerances are tight, with little room for variability in fabrication. Each component must be precisely aligned, so that the light is properly redirected from mirrors 16a, 16b, and 16c. Because each laser reflects off a different mirror, thermal variations between the mirrors 16a, 16b, and 16c adversely affect the alignment of the system during operation. Significantly, for practical reasons, this type of solution allows only a restricted number of lasers, three or fewer, to be combined. The aspect ratio of the composite output beam is fixed by the design of the combining system.
Other proposed solutions for beam shaping and combination, such as using angled light pipes or using various arrangements of combining mirrors fall short of what is needed and impose other constraints, such as by restricting the number and arrangement of lasers that can be combined, or by fixing the aspect ratio of the resulting beam. These conventional solutions for combining separate light beams for laser pump light fail to address the problem of potential damage from feedback light generated by the fiber laser itself. The laser light that is generated at high power in the laser is at a different wavelength from the pump laser wavelength, typically a longer wavelength. Even where a small amount of this fiber laser light finds its way back to the pump lasers, damage to the pump lasers can occur. To compensate for this problem, manufacturers of pump laser diode modules routinely add one or more filters at the output of the pump module in order to attenuate any feedback from the fiber laser. FIG. 1A shows filter 30 provided along the path of composite beam 28 for this purpose, attenuating and blocking feedback light FB. This solution, however, adds cost and components to the design of the pump light module.
In general, the more effective the filter attenuation for unwanted wavelengths and transmission for desired wavelengths, the more complex and high cost the filter. Filters that have particularly sharp transitions between transmission and reflection can also exhibit more pronounced “ringing” or ripple over unwanted wavelengths. In addition, degradation of filter performance over time, such as due to the high energy levels concentrated over small regions of the filter surface, makes this solution less than satisfactory in some applications, and leads to reduced component life.
Thus, it can be seen that there is a need for a method and apparatus for spatially combining light sources, where the method and apparatus are suitable for use with a variable number of laser diodes or other laser sources and help to solve the problem of reducing or eliminating feedback light from a fiber laser or other pumped laser source.